In sensors of process measurement technology and analysis, optical measuring methods are being used more and more frequently. In these methods, the amplitude and/or phasing of an optical signal changes when it strikes a measurement object, and the changed signal is detected by an optical sensor. An example of the application of an optical measuring method of this type is the measurement of the oxygen content or oxygen saturation of a liquid or a substance, wherein a dye is illuminated by a light signal of a previously determined wavelength, amplitude, and phasing, and the luminescent light reflected from the dye is analyzed. When the oxygen concentration in the dye changes, the fade time of the luminescence and thus the amplitude and phasing of the received optical signal also change. With appropriate calibration, the amplitude and phase of the received optical signal are therefore a measure of the oxygen concentration.
There are essentially two prior-art methods being used today to acquire optical signals in the sensors of processing measurement technology and analysis.
In the first method of the prior art, the device and signal curve of which are shown schematically in FIGS. 1a and 1b, an electrical signal is first generated under the command of a control unit 2 by means of a digital-to-analog converter (DAC) 4 and converted by an LED 6 into an optical signal V1. The optical signal reflected by a dye 8 is converted into an electrical measurement signal V2 in a photodiode 10, detected in its entirety by an analog-to-digital converter (ADC) 12, and finally processed digitally and evaluated in the control unit 2. So that the electrical measurement signal V2 can be acquired as accurately as possible in the ADC, it is necessary for the converter to operate with a high degree of oversampling. The sampling points are represented schematically in FIG. 2 as small, filled circles. For example, in the case of a signal with a frequency of 8 kHz, it is necessary to sample typically at 80-800 kSPS. Such analog-to-digital converters with high oversampling comprise a complex digital logic circuit, and for this reason they consume a relatively large amount of power. In addition, these high oversampling rates produce a large amount of digital data, which must be subjected to further processing by a powerful processor.
In the second known measurement method, for which a sensor is illustrated schematically by way of example in FIG. 2, an electrical measurement signal is generated in a signal generator 11 and converted by an LED 6 into an optical signal V1 as in the case of the first known method. The optical signal V1 is directed at a dye 8; the optical measurement signal reflected by the dye is converted in the photodiode 10 into the electrical measurement signal V2, and this is processed by an analog lock-in amplifier. The lock-in amplifier comprises a phase shifter 14 and two analog mixers or multipliers 16. The low-frequency components of the measurement signal filtered by a low-pass filter 18 are acquired by an analog-to-digital converter (ADC) 12 and evaluated in a control unit 2.
The disadvantage of this second method is that the properties of the analog mixer are strongly influenced by component drift. Such component drift is, however, not acceptable in the case of sensors with built-in circuitry which are used in an expanded temperature range in process measurement technology and which must operate for prolonged periods of time without recalibration. In particular, the use of these types of sensors in zones at high risk of explosion is not possible because of the high power consumption.
U.S. Pat. No. 8,078,246 discloses a sensor for pulsoximetry, in which the electrical measurement signal converted by a photodiode is sent to an input amplifier and then distributed over N measurement channels, each of which processes different wavelengths of the amplified measurement signal. Each measurement channel comprises an analog switch, a low-pass filter, and an analog-to-digital converter, and the output signals of the N measurement channels are evaluated in a control unit. The problem here is the same as that cited in relation to the first known method, namely, that the evaluation of the signal in an individual measurement channel is accurate only when the analog-to-digital converter has sufficiently high resolution and thus a correspondingly high oversampling. The difficulties mentioned above are therefore also encountered with U.S. Pat. No. 8,078,246.
It is therefore the object of the present invention to provide a device and a method for measuring a periodic signal which overcome the disadvantages of the prior art, comprise relatively low power consumption and thus low self-heating, contain simple and low-cost components, and guarantee efficient and accurate measurement. This object is achieved by the present claimed invention. Advantageous embodiments are disclosed and claimed herein.